1. Field of the Invention
The present invention relates to circuitry for controlling a bias voltage for an avalanche photodiode (APD), and a method of adjusting the same. More particularly, the present invention is concerned with optimal control over a bias voltage to be applied to an APD.
2. Description of the Background Art
It is a common practice with an optical communication system to use a photoelectric transducer including an APD which is a specific form of a photosensitive device. The APD is capable of amplifying a signal beam current and essential with an optical fiber communication system dealing with weak optical signals.
The amplification of a signal beam current effected by the APD is usually represented by a multiplication ratio M. To cause the APD to operate efficiently, a DC bias voltage of more than several ten volts should be applied to the APD. The multiplication ratio M and the DC bias voltage are closely related to each other. Two different systems are available for applying a DC bias voltage, i.e., a constant multiplication ratio system and a variable multiplication ratio system. The constant multiplication ratio system uses, e.g., a Zener diode and maintains the multiplication ratio M constant by stabilizing the DC bias voltage. The variable multiplication ratio system maintains the DC bias voltage optimal by executing a broad range of automatic gain control (AGC).
The problem with the variable multiplication ratio system is that the multiplication ratio of the APD is susceptible to the temperature variation of an AGC amplifier and the fluctuation of a power source, resulting in a low signal-to-noise (SIN) ratio. The variable multiplication ratio system and constant multiplication ratio system both have a drawback that some means is necessary for controlling a high voltage generating circuit, e.g., a DC/DC converter or a Cockcroft circuit used to boost an oscillator output signal. Such a high voltage generating circuit must be included in a control loop, scaling up the entire circuitry.
Further, as shown in FIG. 2, the multiplication ratio of the APD increases almost infinitely at breakdown voltages of opposite direction. It is therefore necessary to confine the bias voltage in a range below the breakdown voltage. Moreover, as shown in FIG. 3, the breakdown voltage changes with a change in the ambient temperature. Therefore, it is extremely important from the SIN ratio standpoint to effect temperature compensation and to implement an optimal multiplication ratio at a voltage below the breakdown voltage.